A typical ultrasonic surgical device suitable for ophthalmic procedures consists of an ultrasonically driven handpiece, an attached hollow cutting tip, an irrigating sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubings. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubings supply irrigation fluid to and draw aspiration fluid from the eye through the handpiece assembly.
The operative part of the handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece at its nodal points by relatively inflexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. The nosecone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve. Ultrasonic handpieces and cutting tips are more fully described in U.S. Pat. Nos. 3,589,363; 4,223,676; 4,246,902; 4,493,694; 4,515,583; 4,589,415; 4,609,368; 4,869,715; and 4,922,902, the entire contents of which are incorporated herein by reference.
When used to perform phacoemulsification, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined width in the cornea, sclera, or other location in the eye tissue in order to gain access to the anterior chamber of the eye. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying upon contact the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the bore of the cutting tip, the horn bore, and the aspiration line and into a collection device. The aspiration of emulsified tissue is aided by a saline flushing solution or irrigant that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the outside surface of the cutting tip.
The horn assembly, including both piezoelectric and high endurance limit inert materials, used in ultrasonic handpieces must be carefully tuned for proper operation. As used herein, "tuning" refers to the process of finding and tracking the resonant frequencies of the handpiece operating under loaded or unloaded conditions. Operating the handpiece at a resonant frequency takes advantage of the crystal's energy storage capabilities, which occurs most efficiently at resonance. With proper tuning, the handpiece will store mechanical energy while operating unloaded and release this energy into the material being cut when loaded. As a consequence, for short periods of time, large amounts of energy can be directed into the material by the handpiece itself and not by the power source for the handpiece. This allows the power source to be designed to provide only the steady state power requirement of the transducer and not the loaded transients which can be many times higher.
Conventional tuning and control systems determine the series and parallel resonant frequencies under a controlled loading condition, often in water, before the handpiece is subjected to loads encountered during surgery. The handpiece is excited over a range of frequencies, one frequency at a time. The response of the handpiece to each frequency, measured as the admittance (the ratio of the drive current to the drive voltage), is recorded. A typical admittance versus frequency relationship of a typical handpiece in this relatively lightly loaded pre-surgery condition is illustrated in FIG. 1. The maximum recorded admittance (Y.sub.s) corresponds to the series resonance (f.sub.s) and the minimum admittance (Y.sub.p) corresponds to the parallel resonance (f.sub.p). Driving the handpiece with a power signal at the series resonance results in the most efficient conversion of electrical to mechanical energy.
However, when the handpiece is mechanically loaded, as during surgery, the shape of the admittance versus frequency curve changes and thus the characteristic series and parallel resonant frequencies change. Curve "A" in FIG. 2 represents the characteristic admittance versus frequency curve shown in FIG. 1 for a lightly loaded handpiece. Curve "B" represents, for example, the admittance v. frequency curve when the same handpiece is mechanically loaded. As can be seen, curve "B" has shifted right (f.sub.s,B &gt;f.sub.s,A) and the maximum admittance of curve "B" is lower than the maximum admittance of curve "A," and the minimum admittance of curve "B" is higher than the minimum admittance of curve "A." Certain mechanical loading conditions can also shift curve "A" toward lower frequencies (to the left).
Curve "C" represents the admittance v. frequency curve for the same handpiece when the temperature of the crystal within the handpiece has increased somewhat over room temperate. Curve "C" has shifted generally upward and leftward (i.e., higher admittance and lower frequency values) relative to curve "A." If the power signal were delivered to the handpiece at the originally determined series resonance, for example, the efficiency of the power signal drops off dramatically. Thus, subjecting the handpiece to loading without adjusting the frequency of the power signal reduces the efficiency of the power signal.
One approach to tuning a handpiece in real time during surgery employs using a power signal at a frequency that is the average of the series and parallel resonant frequencies in the relatively unloaded pre-surgery condition and adjusts the frequency of the power signal so that a constant admittance is maintained. This type of system is more fully described in U.S. Pat. No. 5,431,664 ("the '664 patent"), which is incorporated by reference. Briefly, systems of this type tune the handpiece based on a constant admittance value (Y.sub.0), which is determined as the average of the maximum and minimum admittances by the equation ##EQU1## (Some commercially available constant-admittance control systems fix Y.sub.0 at a point other than the average, for example, Y.sub.0 =0.3Y.sub.s +0.7Y.sub.p). As the handpiece is loaded under many types of conditions, the admittance v. frequency curve shifts along the frequency axis. Such a shift is shown, for example, by curve "A" and curve "B" in FIG. 2, with curve "B" representing a possible response of a handpiece to mass reactive loading. The control system described in the '664 patent adjusts the frequency of the drive signal to maintain the admittance at Y.sub.0.
While this type of tuning and control system is effective over a relatively broad range of loading conditions, the handpiece is never operated at the series resonance and thus some efficiency is lost. Moreover, shifts in the admittance versus frequency curves along the admittance axis, such as those shown by curve "C" in FIG. 2, may render this type of constant-admittance control system ineffective. The tuning admittance (Y.sub.0) is not found on any portion of curve "C," which may describe the response of a mechanically loaded and heated handpiece.
Control systems that adjust the drive frequency to coincide with the series resonance typically use phase locked loop circuits with very narrow tuning ranges. Thus, a need continues to exist for a method and control system capable of tuning an ultrasonic handpiece to its series resonance when the handpiece is subjected to widely varying load conditions.